Field of the Invention
The present invention relates to reducing the sulfur content from oxidized sulfur-containing hydrocarbon compounds, e.g, formed by oxidative desulfurization of sulfur-containing hydrocarbons.
Description of Related Art
Crude oil is the world's main source of hydrocarbons used as fuel and petrochemical feedstock. While compositions of natural petroleum or crude oils are significantly varied, all crudes contain sulfur compounds and most contain nitrogen compounds which may also contain oxygen, but oxygen content of most crude is low. Generally, sulfur concentration in crude is less than about 5 percent, with most crude having sulfur concentrations in the range from about 0.5 to about 1.5 percent. Nitrogen concentration is usually less than 0.2 percent, but it may be as high as 1.6 percent.
Crude oil is refined to produce transportation fuels and petrochemical feedstocks. Typically fuels for transportation are produced by processing and blending of distilled fractions from the crude to meet the particular end use specifications. Because most of the crudes available today in large quantity are high in sulfur, the distilled fractions must be desulfurized to yield products which meet performance specifications and/or environmental standards.
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sulfur-containing sour crude oil poses health and environmental problems. The sulfur compounds are converted to sulfur oxides during the combustion process and produce sulfur oxyacids and contribute to particulate emissions.
Oxygenated fuel blending compounds and compounds containing few or no carbon-to-carbon chemical bonds, such as methanol and dimethyl ether, are known to reduce smoke and engine exhaust emissions. However, most of such compounds have high vapor pressure and/or are nearly insoluble in diesel fuel, and they have poor ignition quality, as indicated by their cetane numbers. For instance purified diesel fuels by chemical hydrotreating and hydrogenation to reduce their sulfur and aromatics contents, also causes a reduction in fuel lubricity. Diesel fuels of low lubricity may cause excessive wear of fuel pumps, injectors and other moving parts which come in contact with the fuel under high pressures. Mid distillates, a distillate fraction that nominally boils in the range 180-370° C. are used for fuel or a blending component of fuel for use in compression ignition internal combustion engines (diesel engines). They usually contain from about 1 to 3 percent by weight sulfur. The specification for mid distillate fraction have been reduced to 5-50 part per million weight (ppmw) levels from 3000 ppmw level since 1993 in Europe and United States.
In order to comply with these regulations for ultra-low sulfur content fuels, refiners have to make fuels having even lower sulfur levels at the refinery gate so that they can meet the stringent specifications after blending at the gate. Refiners must choose among the processes or crude oils that provide flexibility that ensures future specifications are met with minimum additional capital investment, in many instances by utilizing existing equipment. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new grassroots production facilities are constructed. However, many existing hydroprocessing facilities, such as those using relatively low pressure hydrotreaters, represent a substantial prior investment and were constructed before these more stringent sulfur reduction requirements were enacted. It is very difficult to upgrade existing hydrotreating reactors in these facilities because of the comparatively more severe operational requirements (i.e., higher temperature and pressure) to obtain clean fuel production. Available retrofitting options for refiners include elevation of the hydrogen partial pressure by increasing the recycle gas quality, utilization of more active catalyst compositions, installation of improved reactor components to enhance liquid-solid contact, the increase of reactor volume, and the increase of the feedstock quality.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for and are being operated at, relatively mild conditions (i.e., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of 180° C. to 370° C.).
With the increasing prevalence of more stringent environmental sulfur specifications in transportation fuels mentioned above, the maximum allowable sulfur levels are being reduced to no greater than 15 ppmw, and in some cases no greater than 10 ppmw. This ultra-low level of sulfur in the end product typically requires either construction of new high pressure hydrotreating units or a substantial retrofitting of existing facilities, e.g., by incorporating gas purification systems, reengineering the internal configuration and components of reactors, and/or deployment of more active catalyst compositions.
Sulfur compounds can be classified into four groups according to their hydrodesulfurization reactivity described by the pseudo-first-order rate constants. See, e.g., X. Ma et al., Ind. Eng. Chem., 1994, 33, 218; X. Ma et al., Ind. Eng. Chem. Res., 1995, 34, 748. These groups are:
The first group is predominantly alkyl benzothiophenes (BTs); the second, dibenzothiophenes (DBTs) and alkyl DBTs without alkyl substituents at the 4- and 6-positions; the third group, alkyl DBTs with only one alkyl substituent at either the 4- or 6-position; the fourth group, alkyl DBTs with alkyl substituents at the 4- and 6-positions. The relative hydrodesulfurization rate constant for each of the four groups is 36, 8, 3, and 1, respectively.
When the total sulfur content is reduced to 500 ppmw, the main sulfur compounds remaining in the hydrotreated effluent are the third and fourth groups. When the total sulfur content is reduced to 30 ppmw, the sulfur compounds remaining are only the fourth group sulfur compounds, indicating that the lower sulfur content organosulfur compounds have lower hydrodesulfurization reactivity. See D. D. Whitehurst et al., Catalysis Today, 1998, 45, 299.
Consequently, these species from the third and fourth groups are referred to as refractory sulfur compounds. Both steric hindrance and electronic density factors contribute to the low reactivity levels of 4- and 6-alkyl substituted DBTs in hydrodesulfurization process. See X. Ma et al. (1995); M. Daage et al., J. Catal., 1994, 194, 414.
The economical removal of refractory sulfur-containing compounds is therefore exceedingly difficult to achieve, and accordingly removal of sulfur-containing compounds in hydrocarbon fuels to an ultra-low sulfur level is very costly by current techniques. When previous regulations permitted sulfur levels up to 500 ppmw, there was little need or incentive to desulfurize beyond the capabilities of conventional hydrodesulfurization, and hence the refractory sulfur-containing compounds were not targeted. However, in order to meet the more stringent sulfur specifications, these refractory sulfur-containing compounds must be substantially removed from hydrocarbon fuels streams.
Compared with conventional catalytic hydrodesulfurization, oxidative desulfurization (ODS) can be performed under mild conditions, i.e., relatively low temperature and under atmospheric pressure conditions. ODS typically uses an oxidizing agent, such as hydrogen peroxide, organic peroxide, peracid, ozone, air and oxygen, in addition to an oxidation catalyst. In the oxidation process, the divalent sulfur atom of refractory sulfur compounds (condensed thiophene) is oxidized by the electrophilic addition reaction of oxygen atoms to form the hexavalent sulfur of sulfones. The chemical and physical properties of sulfones are significantly different from those of the hydrocarbons in fuel oil. Therefore, sulfones can be removed by conventional separation methods such as filtration, solvent extraction and adsorption. An effective ODS process which has been shown to decrease sulfur in transportation fuel from 1100 ppm to 40 ppmw, is described in Al-Shahrani et al. WO/2007/103440 and in Al-Shahrani et al. Applied Catalysis B, V. 73, No. 3-4, p. 311 (2007). ODS is considered a promising substitute or supplement to hydrodesulfurization for deep desulfurization of transportation fuels.
The compositions of common sulfides in fuel oil and their respective sulfones are tabulated in Table 1:
TABLE 14,6-DimethylDBT4-Methyl4-Methyl DBT4,6-DimethylDBT SulfoneSulfoneDBTSulfoneDBT(4,6-DBT(DBTO2)(4-MDBT)(4-MDBTO2)(4,6-DMDBT)DMDBTO2)CH %82.5872.1883.8172.1684.8873.76S %17.4213.9216.1913.9415.1213.14O %013.90013.90013.10
Sulfones formed by ODS of diesel fuels are complex mixtures that vary based on the crude source and other factors, including DBT sulfone along with several alkyl substituted DBT sulfones, such as 4-MDBT sulfone, 4,6-DMDBT sulfone, 1,4-DMDBT sulfone, 1,3-DMDBT sulfone, TriMDBT sulfone, TriEDBT sulfone, and C3DBT sulfone. The structures of certain sulfones found in ODS treated sulfones are given below.

Unlike hydrodesulfurization, in which desulfurized products remain with fuel oil and organic sulfur is converted into gaseous H2S that leave the product, i.e., fuel oil mixture, oxidation products including sulfones (collectively referred to as “oxidized sulfur-containing hydrocarbons” or “oxidized sulfur-containing hydrocarbon compounds”) as formed by ODS remain in the hydrocarbon mixture and must be separated from the product. Various attempts have been made to dispose of oxidized sulfur compounds formed by oxidative desulfurization. These techniques include conventional extraction, distillation and/or adsorption, and conversion in conventional refining processes including delayed coking, FCC, gasification and solvent deasphalting.
In U.S. Pat. No. 6,277,271, incorporated herein by reference, there is disclosed a process for the desulfurization of hydrocarbonaceous oil wherein a stream, composed of hydrocarbonaceous oil and a recycle stream containing oxidized sulfur compounds, is contacted with a hydrodesulfurization catalyst in a hydrodesulfurization reaction zone to obtain low level of sulfur. The resulting hydrocarbonaceous stream is then contacted in its entirety with an oxidizing agent in an oxidation reaction zone to convert the residual sulfur compounds into oxidized sulfur compounds. After decomposing the remaining oxidizing agent, the oxidized sulfur compounds are removed resulting in a stream containing these latter and a stream of hydrocarbonaceous oil having a reduced concentration of oxidized sulfur compounds. At least a portion of the oxidized sulfur compounds are recycled back to the hydrodesulfurization reaction zone to increase the hydrocarbon recovery from the process. However, some of the sulfones compounds formed are reduced back to the initial sulfur compounds still leaving the sulfur disposal problem not fully resolved.
U.S. Pat. No. 6,087,544, incorporated herein by reference, discloses a process to produce distillate fuels having a sulfur level below the distillate feedstream. The distillate feedstream is first fractionated into a light fraction which contains only from about 50 to 100 ppmw of sulfur, and a heavy fraction. The light fraction is then sent to a hydrodesulfurization reaction zone to remove substantially all of the sulfur therein. Finally part of the desulfurized light fraction is then blended with half of the heavy fraction to produce a low sulfur distillate fuel. However, not all the distillate feedstream is recovered to obtain a low sulfur distillate fuel product.
U.S. Pat. No. 6,171,478, incorporated herein by reference, discloses an integrated process in which the hydrocarbonaceous feedstock is first contacted with a hydrodesulfurization catalyst in a hydrodesulfurization reaction zone to reduce the sulfur level to a low sulfur level. The resulting hydrocarbonaceous stream is then sent in its entirety to an oxidation zone containing an oxidizing agent where the residual sulfur is converted into oxidized sulfur compounds under mild conditions. After decomposing the residual remaining oxidizing agent, the oxidized sulfur compounds produced are then extracted using a solvent resulting in a stream containing the oxidized sulfur compounds and a hydrocarbonaceous oils stream having a reduced concentration of oxidized sulfur compounds. A final step of adsorption is carried out on the latter to reach ultra-low sulfur levels.
In WO2002/18518, incorporated herein by reference, a two stage desulfurization process is placed downstream of a hydrotreater. After having been hydrotreated in a hydrodesulfurization reaction zone the entire distillate feedstream is then sent to an oxidation reaction zone to undergo an aqueous formic acid based hydrogen peroxide biphasic oxidation to convert the thiophenic sulfur compounds to the corresponding oxidized compounds, i.e. sulfones. Some of the sulfones end up in the aqueous oxidizing solution during the oxidation reaction and are further removed by a subsequent phase separation step. The oil phase containing the remaining sulfones is finally subjected to a liquid-liquid extraction step. No mention is made about the fate of the sulfones.
WO2003/014266, incorporated herein by reference, discloses a process for the removal of the sulfur from a hydrocarbon stream. The hydrocarbon stream containing the sulfur compounds is sent to an oxidation reaction zone where the organic sulfur compounds are oxidized into the corresponding sulfones using an aqueous oxidizing agent. After separating the aqueous oxidizing agent from the hydrocarbon phase the resulting hydrocarbon stream is sent to the hydrodesulfurization step. The resulting hydrocarbon is substantially sulfur reduced.
WO2006/071793, incorporated herein by reference, discloses a process that reduces the sulfur and/or nitrogen content of a distillate feedstock to produce a transportation fuel or blending components for transportation fuel. The hydrotreated feedstock is contacted with an oxygen-containing gas and a titanium-containing mesoporous oxidation catalyst in an oxidation/adsorption zone to convert the sulfur compounds into the corresponding sulfones that are adsorbed onto the catalyst. No mention is made about the fate of the sulfones.
U.S. Patent Publication No 2005/0150819A1, incorporated herein by reference, discloses a process for removing sulfur compounds found in a hydrocarbon stream. The sulfur compounds are first introduced in a concentration zone for increasing their concentration via e.g. complexation with ammonium complexes, adsorption or extraction and then separated from the sulfur depleted petroleum feedstock. A selective oxidation of the separated sulfur compounds is then performed in the gas phase using air or oxygen in the presence of a supported catalyst into valuable oxygenated products and sulfur deficient hydrocarbons.
In U.S. Pat. No. 6,368,495, incorporated herein by reference, a process effective for the removal of organic sulfur compounds from liquid hydrocarbons is disclosed. The process more specifically addresses the removal of thiophenes and thiophene derivatives from a number of petroleum fractions, including gasoline, diesel fuel, and kerosene. In the first step of the process, the liquid hydrocarbon is subjected to oxidation conditions in order to oxidize at least some of the thiophene compounds to sulfones. Then, these sulfones can be catalytically decomposed to hydrocarbons (e.g. hydroxybiphenyl) and volatile sulfur compounds (e.g. sulfur dioxide). The hydrocarbon decomposition products remain in the treated liquid as valuable blending components, while the volatile sulfur compounds are separable from the treated liquid using well-known techniques such as flash vaporization or distillation.
Other sulfone conversion techniques include those described in US20120055849, US20120055845, US20120055844, US20120055843, which are commonly owned with this application, and describe sulfone conversion by gasification, delayed coking, fluid catalytic cracking (FCC) and solvent deasphalting, respectively. In addition US20130015104, which is also commonly owned with this application, describes sulfone decomposition by super electron donors.
With the steady increase in demand for hydrocarbon fuels having an ultra-low sulfur level, a need exists for an efficient and effective process and apparatus for desulfurization while maximizing product yield.
Accordingly, it is an object of the present invention to effectively reducing the concentration of oxidized sulfur compounds from a mixture of liquid hydrocarbons and the oxidized sulfur-containing hydrocarbon compounds while minimizing loss of hydrocarbon.